1. Field of the Invention
The present invention relates generally to the field of manufacturing flat gaskets produced by alternating stacks of flexible graphite and perforated metal foils, which are capable of withstanding high-temperature conditions of 300° C. or more, for example, without undergoing deterioration in their quality, even under very high clamping stresses.
2. Description of Related Art
Flexible graphite is fabricated by thermal expansion of graphite (most often in the form of flakes), into which atoms or molecules have been inserted following attack in an acid medium; the material thereby obtained has a very low mass density and possesses the property of self-agglomerating without any binder, via a simple mechanical effect. In this way, a flexible or semi-rigid material in the form of rolls or plates is obtained by rolling or compression.
Flexible graphite foils have been used for a long time for the manufacture of flat gaskets. Such flat gaskets are used, for example, in chemical or petrochemical industry plants, for transporting hot and corrosive fluids, as well as in thermal or nuclear power plants, for transporting pressurized water vapour. The use of a flat gasket is shown schematically in FIG. 1. Two metal clamps (1, 2) join together two tubular pipes (5, 6) thus forming a pipeline. Clamping of the two metal clamps (1, 2) by means of bolts (3) situated along the periphery of the assembly enables pinching of the flexible graphite foil (4) acting as a gasket. The flexibility properties and the deformation capability of flexible graphite enable it to conform to the surfaces opposite the metal clamps and to ensure a proper seal between the interior of the pipeline (a) and the external environment (b). The thermal stability and high chemical inertness qualities of flexible graphite, particularly with respect to organic or acidic liquids, have made it the material of choice in many situations.
Thus, there are three determining characteristics with regard to the quality of flat gaskets: the sealing capability (expressed in the form of a leakage rate measured under normalised conditions), the maximum degradation temperature of the materials forming the gasket, and finally maintenance of the mechanical properties of the gasket structure within the operating temperature range of the materials forming it. The characteristics of the gasket must, on the one hand, always enable its adaptation to the surfaces against which it is pressed and, on the other hand, its creep resistance, in order to maintain the clamping pressure of the clamps over the course of time and temperature cycling, and to do so in order to guarantee the seal over time.
Despite the fact that certain qualities of flexible graphite withstand an air temperature as high as 500° C., or even 550° C., flexible graphite foils suffer from several disadvantages. They are difficult to handle, they tear relatively easily, and it is difficult to produce them in heavy thicknesses. The producers of flexible graphite foils have thus developed multi-material stacks, generally alternating stacks of metal foils and flexible graphite, in order to make it more practical to use the gaskets and to make them more mechanically strong. Today, it is very common to use a gasket consisting of a stack such as the one described in FIG. 2, wherein two flexible graphite foils (10, 11) are bonded to a middle metal foil (insert) (12). These gaskets are also stressed in the direction parallel to the layers that form them, due to the poorly distributed, heavy compressive stress over the entire surface of the gasket; this phenomenon is called “the toe-in of the clamp.” Thus, they may have a creep problem, particularly at a high-operating temperature, when the thermal expansion deforms the geometry of the clamp. Creep is therefore likely to limit their lifespan and the seal of the system of which they are a part.
According to this principle, and primarily in order to further improve the mechanical strength of the gasket, numerous solutions have been proposed. Depending on the thicknesses of the gasket, these solutions involve stacks of 3, 5, 7 or more layers, various materials for the reinforcing foils (various metals, solid foils or perforated foils, or even grids), and various solutions to ensure the mechanical bond between the flexible graphite and the reinforcing foil. Among these bonding solutions, the two main technologies used can be cited: either a gluing or anchoring of mechanical holding means into the graphite foils. These mechanical holding means can be dome-like structures or spurs resulting from the perforation of a thin plate or metal foil with the aid of an awl (see the patent application FR 2 625 281 (Dana Corporation)).
In this association of materials of the type involving flexible graphite foils attached to a rigid metal structure, the flexible graphite foils ensure the function of deformability/conformation to the contact and sealing surfaces, while the metal reinforcements provide the advantage of strength to the entire assembly, and thereby enable easy handling (even for large-sized gaskets) and provide the assembly with a much improved creep resistance.
To fasten a flexible graphite foil onto a metal plate or foil, glues or adhesives can be used conventionally, but the latter cannot guarantee mechanical strength beyond 300° C. The patents EP 616 884, U.S. Pat. No. 5,509,993 and U.S. Pat. No. 6,962,349 (Sigri Great Lakes Carbon AG) disclose the use of substances that alter the interface between the metal and the graphite, but that are not glues, such as certain organosilicon compounds, perfluorinated compounds or metallic soaps. These products are adhesion promoters; they must be applied in a thickness of a few nanometers. In this way, a layer of metal is fastened to a layer of graphite without glue, using a hot-pressing technique, typically at a temperature ranging between 150° and 300° C. (see U.S. Pat. No. 6,258,457 (SGL Technik GmbH)). This technique, however, is very expensive to implement because it is not very productive, and it does not guarantee a sufficient degree of mechanical strength for the assembly beyond 400° C.
Another technical approach uses mechanical holding means that can be obtained by creating numerous dome-shaped perforations in the metal foil (see European Patent Application EP 0 640 782 A2 (Tako Payen S.p.a.), French Patent Application 2 625 281 (Dana Corporation), U.S. Pat. No. 4,723,783 (Dana Corporation), U.S. Pat. No. 6,258,457 (SGL Technik GmbH)). However, as taught by U.S. Pat. No. 5,509,993 cited above, the perforation of the dome-shaped plates causes local stresses in the plate, which can lead to ruptures under load. However, the stacks of flexible graphite foils attached to metal plates still have some weak points. First of all, the perforated metal reinforcements in strip thicknesses greater than or equal to 100 μm make it more difficult to cut out the gaskets, an operation that makes it possible to obtain the desired geometries from flat foils. In order to limit this disadvantage, the common practise is to limit the number of perforated metal reinforcements and to also limit their thickness. A single metal reinforcement is typically used, and sometimes two, for a total thickness of 3 mm. The strip thicknesses are more frequently close to 100 micrometers.
In summary, the solutions for bonding between layers by gluing on the one hand introduce an element (the glue) whose temperature-related strength is limited. Furthermore, they require production methods that are more difficult to implement than the simple roll-bonding used to bond together a perforated foil and a flexible graphite foil. On the other hand, glue-less assembly methods also exist, but these methods are likewise complex, because they make use of hot-pressing methods as well as the application of a very slight thickness of chemical products that alter the surface.
While roll-bonding can be easily understood as an operation producing a continuous “sandwich” of materials, gluing will require surface coating, drying and, most often, especially for glues capable of performing at temperatures of the order of 300° C., a heat treatment in order to stabilise the glues. This series of operations is either carried out in successive steps, or by means of a complex series of continuously operating equipment.
In any event, roll-bonding with a perforated plate seems to be the most economical continuous assembly method, but has significant disadvantages such as the difficulty of cutting via conventional means.
Generally, when the operating temperature exceeds 400° C., and when the pressures of the fluids being sealed off are too great, flat gaskets cut out of composite flexible graphite-based plates must be replaced by more reliable but also more costly solutions; however, these solutions are less dimensionally flexible, such as spiral gaskets, serrated gaskets and other metal gaskets.
Thus, a problem to which the present invention attempts to respond is to propose a new method for manufacturing plates and/or gaskets comprising an alternating stack of layers of flexible graphite and metal foils enabling easy cutting and easy and economical continuous production, and which has very good mechanical strength up to temperatures and pressures that have until now been impossible for flat flange gaskets, while at the same time guaranteeing a seal according to new and/or existing standards aiming to limit fugitive emissions of gases that are ecologically dangerous to the atmosphere.